Its a built in gear counter. MoCo put it there to reduce phantom 6th shifts. If you hear the whine, ya got one more. If you don't, well then your in 5th. And they say the Sportster is outdated. Name one more bike that has this kind of technology.

i don't think it's anything to worry about. Mine does it too (only in 4th also) My cousins XL does the same thing. I was on a ride with another guy who had a Sportser, his did it too. The years ranged from '96 to '04.

i have one question though do the 06 models have the same issue ( they have helical cut gears)??

So far it's an observation and not an issue. I have been doing some searches and it seems 04-05 touring bikes also whine in 4th gear. Some of them have 20K+ mi. on them and all is still fine. Since the 06 bikes have completely different transmissions, it is highly unlikely their transmissions would behave the same.

Its a built in gear counter. MoCo put it there to reduce phantom 6th shifts. If you hear the whine, ya got one more. If you don't, well then your in 5th. And they say the Sportster is outdated. Name one more bike that has this kind of technology.